Electron emitter comprising photoconductive and low work function layers



ct. ii, 166 H. KANTER ELECTRON EMITTEB COMPRISING PHOTOGONDUC'IIVE AND LOW WORK FUNCTION LAYERS Filed Aug. 23, 1962 I III/l 1/1 I!!! I III/I H 4 I III! L f 7/ L \m g INVENTOR HelmuT Kanter M WITNESSES.

ATTORNEY United States Patent ELECTRON EMITTER COMPRISING PHOTOCON- DUCTIVE AND LOW WORK FUNCTION LAYERS Helmut Kanter, Mouroeville, Pa., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Aug. 23, 1962, Ser. No. 219,022 7 Claims. (Cl. 313-94) The present invention relates to light sensitive electron emitting devices, and more particularly to electron emitting devices utilizing high gain photoconductors.

Light sensitive devices such as imaging tubes and television pick-up tubes must have high sensitivity in order to convert light signals to electrical signals at high resolution. Photoconductors have a high sensitivity because of their, so-called, internal high gain. That is, if a photoconductor is placed between electrically conducting electrodes and an electric field is placed thereacross, an incident photon will cause a large number of majority carriers to flow between the electrodes. Except for the carriers produced by the incident photon, by far the largest number of carriers originate from the replenisher electrode. The gain of the device is given by the ratio of minority carrier life time to the transit time of the majority carrier. Gains in the order of 1000 or more have been obtained. In this particular application the majority carriers are electrons, flowing towards a thin positive electrode. The sensitivity and the gain of the photoconductive device, in large part, depend upon the thickness and material of the positive electrode especially when emission into a vucuum is desired.

It is therefore an object of the present invention to provide a new and improved photoconductive electron emissive device having a high gain and high sensitivity.

It is a further object of the present invention to provide a new and improved photoconductive electron emissive device having a high gain and high sensitivity and utilizing a low work function emitter electrode.

Broadly, the present invention provides a photoconductive electron emissive device in which a layer of photoconductive material is sandwiched between a layer of light transparent electrically conductive material and a layer comprising a low work function material, so that, electrons, under the influence of an electric field, may be readily transmitted through the device into a vacuum to provide a source of electrons in response to light.

These and other objects will become more apparent when considered in view of the following specification and drawings in which:

FIGURE 1 is a schematic diagram embodying the teachings of the present invention; and

FIG. 2 is an energy versus distance diagram used in aid in the explanation of the operation of the present invention.

Referring to FIG. 1, an electron discharge tube structure is shown having an envelope T, which may be evacuated to form a vacuum v therein. An anode electrode A, having an external terminal a, is disposed at one end of the tube. A grid electrode G, having an external terminal g, is also disposed Within the envelope T. The photoconductive cathode is disposed at the other end of the tube envelope T to convert light energy into electrical energy. A transparent electrically conducting layer M is evaporated adjacent the wall W of the envelope T. The wall W may be glass and transmissive to light. The layer M may for example comprise indium and be such a thickness to be transmissive to light and yet electrically conducting. Deposited by evaporation onto the transparent metal layer M is a layer PC of photoconductive material, which for example may be cadium sulfide. The

3,278,782 Patented Oct. 11, 1966 photoconductive layer PC may be of a thickness between 1 and microns. Onto the photoconductive layer PC is deposited an electrode L by, for example, evaporation. The electrode L may comprise an alkali metal, such as potassium, or an earth alkali metal, such as barium. The layer L should be of the order of Angstroms. Or, the layer L may comprise a metal layer that is oxidized at its surface to provide a low work function, such as barium oxidized to form barium oxide. Also the layer L could be formed by alloying a metal layer with another material, for example, gold alloyed with barium. Another method of providing the low work function layer L would be to deposit a monolayer of a low work function material on a metal surface, such as, a monolayer of cesium deposited on a layer of antimony. There are, of course, other methods that could be used to provide the low work function layer L, such as disposing a mesh in a matrix pattern on the layer PC. A source of direct potential V is connected with its positive terminal to the layer L and with its negative terminal connected to the transparent metal layer M. Thus, an accelerating electric field is applied across the photoconductive layer PC.

Referring to FIG. 2, with no light being supplied to the photoconductive layer PC the photoconductor is in its dark state, that is, only few electrons are emitted into the vacuum v corresponding to the dark current within the PC. However, when the photoconductor is illuminated, photons of light are supplied to the photoconductor PC through the transparent layer M, with the bias potential being applied across the photoconductor PC. The Fermi energy level of the metal M is shown to be Efm. The Fermi level of the layer L is substantially lower at a level Efl, with the Fermi level Efpc of the photoconductive layer PC being between that of the layer M and the layer L. By using the low work function layer L, the vacuum barrier is substantially reduced to Efl to allow relatively low energy electrons from the conduction band of the photoconductive layer PC to penetrate the layer L and then be emitted into the vacuum v. When the photoconductor PC is illuminated by supplying photons through the transparent layer M to the photoconductive layer PC, electron-hole pairs will be created in the photoconductor PC, with electrons in the conduction band, at an energy above the forbidden region, flowing toward the positive electrode L and holes in the valence band of the photoconductor flowing toward the negative electrode M. Thus, a current will exist across the photoconductor PC until the holes have diffused into the electrode M. The electrons in the photoconductor will drift toward the low work function electrode L being accelerated in the conduction band by the electric field. Because the mean free path of low energy electrons in metals is quite large, up to several hundred angstroms, a portion of the electrons entering the layer L will be able to penetrate the film L and to escape from the surface into the vacuum v. Once electrons are in the vacuum v, they may be controlled by the operation of the grid electrode G and the anode electrode A in the usual manner. Furthermore, by using :a thin film L, with a low work function, a high transfer ratio would result, that, is, the number of electrons that escape from the layer L is a high percentage of those being introduced into the layer L from the photoconductor PC. Assuming that the transfer ratio would be only 0.01, using a photoconductor with an internal gain of 1000, about ten electrons per photon would be able to escape into the vacuum of the tube T. Conventional photo-emitters emit an 'average of about one electron per ten photons. Thus, a substantial increase in gain and sensitivity is achieved with the effect being that an internal pre- 3 amplifier is provided within the photocathode electron emissive device itself.

Although the present invention has been described with a certain particularity, it should be understood that the present disclosure has been made only by way of example and that numerous changes in the details of fabrication, the materials used and the combination and arrangement of elements may be resorted to without departing from the scope and the spirit of the present invention.

I claim as my invention:

1. An electron emissive device including, a first layer comprising a light transparent electrically conducting material, a second layer comprising a photoconductive material disposed adjacent said first layer, a third layer disposed contiguous to said second layer and comprising an electrically conductive material having a low work function surface and having a thickness such that the transfer ratio of electrons passing through said third layer compared to the electrons entering the layer is high, and a potential source connected across said third layer and said first layer to :apply a potential to said device to accelerate electrons thereacross.

2. A photoconductive electron emissive device including, a first layer comprising a light transparent electrically conducting material, a second layer comprising a photoconductive material disposed adjacent said first layer, a third layer disposed contiguous said second layer, said third layer comprising a low work function electrically conductive alkali metal having a thickness such that the transfer ratio of electrons passing through said third layer compared to the electrons entering said third layer is high, and a potential source connected across said third layer and said first layer to apply a potential to said device to accelerate electrons thereacross.

3. An electron emissive device including, a first layer comprising a light transparent electrically conductive material, a second layer comprising a photoconductive material disposed adjacent said first layer, a third layer disposed contiguous said second layer, said third l-ayer comprising an electrically conductive metallic material coated with a monolayer of an alkali metal and having a low work function, and a potential source connected across said third layer :and said first layer to apply a potential to said device to accelerate electrons thereacross.

4. A photoconductive electron emissive device including, a first layer comprising a light transparent electrically conductive material, a second layer comprising a photoconductive material disposed adjacent said first layer, a third layer disposed contiguous said second layer, said third layer comprising a low work function electrically conductive alkali metal and having a thickness of less than 500 Angstroms, and a potential source con- 4 nected across said third layer and said first layer to apply a potential to said device to accelerate electrons thereacross.

5. A photoconductive electron emissive device including a first layer comprising a light transparent electrically conducting material, a second layer comprising a photoconductive material being disposed adjacent said first layer, and a third layer disposed adjacent said second layer, said third layer comprising a metallic electrically conductive material oxidized at its structure to have a low work function, and a potential source connected across said third layer and said first layer to apply a potential to said device to accelerate electrons thereacross.

6. An electron discharge device responsive to light including, an anode, a grid, and a photoconductive cathode, said cathode including a first layer comprising a light transparent electrically conductive material, a second layer comprising a photoconductive material disposed adjacent said first layer, a third layer disposed contiguous said second layer, said third layer comprising an electrically conductive material having a low work function surface and having a thickness such that the transfer ratio of electrons passing through the layer compared to electrons entering the layer is high, and a potential source connected across said third layer and said first layer to apply a potential to said device to accelerate electrons thereacross.

7. An electron discharge device responsive to light including, an anode, a grid and a photoconductive cathode, said cathode includiug a first layer comprising a light transparent electrically conducting material, a second layer comprising a photoconductive material disposed adjacent said first layer, a third layer disposed contiguous said second layer, said third layer comprising a low work function electrically conductive alkali metal having a thickness of less than 500 Angstroms, and :a potential source connected across said third layer and said first layer to apply a potential to said device to accelerate electrons thereacross.

References Cited by the Examiner UNITED STATES PATENTS 2,850,657 9/1958 McNaney 313-- 2,970,219 1/1961 Roberts et al 31365 X 3,002,101 9/1961 Anderson et al. 3l365 X 3,184,636 5/1965 Dore et al. 313346 X JAMES W. LAWRENCE, Primary Examiner.

JOHN W. HUCKERT, GEORGE N. WESTBY,

V. LAFRANCHI, R. SEGAL, Assistant Examiners. 

1. AN ELECTRON EMISSIVE DEVICE INCLUDING, A FIRST LAYER COMPRISING A LIGHT TRANSPARENT ELECTRICALLY CONDUCTING MATERIAL, A SECOND LAYER COMPRISING A PHOTOCONDUCTIVE MATERIAL DISPOSED ADJACENT SAID FIRST LAYER, A THIRD LAYER DISPOSED CONTIGUOUS TO SAID SECOND LAYER AND COMPRISING AN ELECTRICALLY CONDUCTIVE MATERIAL HAVING A LOW WORK FUNCTION SURFACE AND HAVING A THICKNESS SUCH THAT THE TRANSFER RATIO OF ELECTRONS PASSING THROUGH SAID THIRD LAYER COMPARED TO THE ELECTRONS ENTERING THE LAYER IS HIGH, AND A POTENTIAL SOURCE CONNECTED ACROSS SAID THIRD LAYER AND SAID FIRST LAYER TO APPLY A POTENTIAL TO SAID DEVICE TO ACCELERATE ELECTRONS THEREACROSS. 